Do sputum eosinophils and ECP relate to the severity of asthma?

There is much evidence that eosinophils play an important role in bronchial epithelial damage in asthma by releasing cationic proteins. However, the extent to which eosinophil inflammation relates to indices of asthma severity in chronic stable asthma is still a matter of debate. We studied 46 clinically stable patients with mild to severe chronic asthma (forced expiratory volume in one second (FEV1) 50-126% of predicted value). The clinical severity of asthma was graded from 1 to 4 according to the Aas scoring system. Twelve normal subjects were also studied as controls. Induction of sputum was performed by hypertonic saline to determine differential cell count, and eosinophil cationic protein (ECP) by the so-called "plug technique". The concentration of ECP was measured by a fluoroimmunoassay. Bronchial hyperresponsiveness was recorded by inhaling progressive concentrations of histamine, and the concentration that caused a 20% decrease in FEV1 (PC20) was calculated. Sputum eosinophils (range 0-61%), sputum ECP (range 24-10,800 microg x L[-1]) and serum ECP (range 4-61 microg x L[-1]) were significantly greater in asthmatics than in normal subjects, and distinguished the most severe group with the highest Aas score from the others. Sputum eosinophils and sputum ECP were strongly related to each other. The relationships between sputum or serum ECP and PC20 (range 0.016-7.5 mg x mL[-1]), and between sputum ECP and FEV1 were found to be weak. In conclusion, sputum outcomes of eosinophil activation and serum eosinophilic cationic protein appear to be useful indicators of disease. They do not accurately reflect current clinical or functional indices of asthma severity in chronic stable patients, and might therefore provide complementary data disease monitoring.